1. Field of the Invention
The invention relates to a device with a plasma torch.
2. Description of the Prior Art
Such devices with merely one rod-shaped electrode are used for the welding of light metal and light metal alloys. In order to achieve a high welding speed at deep fusion penetration and narrow seems, the rod-shaped electrode is used as a cathode, and helium is used as a plasma gas. A very hot plasma is obtained which evaporates thin oxide layers. This is not the case in all light metal alloys, however.
In order to enable the welding of such alloys, too, welding is performed with direct current instead of alternating current, or the electrode is connected to the plus pole of the voltage source. Although a continual removal of the oxide layers is ensured and a welded joint is ensured which is free from cavities because the oxide skin is continuously torn open, this advantage is offset by the disadvantage of a welding speed which is reduced by approximately two-thirds as compared with a d.c. helium welding and a considerable increase of the width of the weld seams, resulting in an increased heat influence zone.
It is the object of the present invention to avoid such disadvantages and to provide a device of the kind mentioned above, which allows a high welding speed in difficult alloys, too, and which also ensures that any oxide layers are removed.
This is achieved with a device with a plasma torch, which comprises a receiver made from an electrically non-conducting material, two rod-shaped non-consumable electrodes held in nozzles in the receiver, the electrodes enclosing an acute angle, and a counterelectrode associated with the non-consumable electrodes. A separate voltage source is connected to each non-consumable electrode and the voltage sources are also connected to the counterelectrodes. Each voltage source has a plus pole and a minus pole, one of the non-consumable electrodes being connected to the plus pole and the other non-consumable electrode being connected to the minus pole. The voltage sources supply direct voltage pulses having a level exceeding at least the arc voltage of an arc between the non-consumable electrodes, and mutually locked switching devices prevent a simultaneous voltage supply from the voltage sources to the non-consumable electrodes and supply voltage at different polarization to the non-consumable electrodes. A gas supply is connected to the nozzles.
With such a device, it is possible to connect the two rod-shaped electrodes with different poles of the voltage source. As a result, plasma pulses which are produced with an electrode connected to the plus pole of a voltage source can be used to tear open the oxide layers, and with the subsequent plasma pulses connected with the minus pole of a voltage source and, therefore, produced by an electrode used as a cathode, it is possible to weld the basic material in a clean manner and with a high penetration depth, with very narrow and smooth weld seams being obtained. By locking the respective switching devices, which each only allow voltage pulses of approximately 1 to 5 milliseconds, it is ensured that only one electrode can be charged.
The workpiece to be welded can appropriately be used as a counterelectrode. It is also possible to make the nozzle or the nozzle body from an electrically conductive material and to use the same as a counterelectrode.
In the case of alloys that can be welded more easily, both electrodes may be used as cathodes. This leads to the advantage that the required welding energy can be divided among both electrodes and they can, therefore, be provided in a narrower housing. This allows the production of very narrow receiving means of 9 mm width, for example. With such devices, it is, therefore, also possible to weld in corner zones of workpieces which are difficult to access, which substantially facilitates the structural design of such devices.
The two separate voltage sources may also be controlled with respect to the pulse length and pulse power, thus enabling an adaptation to various requirements.
The ignition of each plasma arc may be made by means of a high-frequency pulse when the level of the voltage of the individual voltage pulses does not exceed the breakdown voltage of the path between the electrode and the respective counterelectrode. The ignition can also be initiated by high voltage pulses which exceed the respective breakdown voltage.
Very favorable conditions for the welding of very difficult alloys are obtained if a workpiece to be welded is connected to one pole of each voltage source, and the non-consumable electrodes are connected to different poles of the voltage sources, the non-consumable electrode connected to the plus pole being disposed at the front, as seen in the direction of welding. It ha s also been proven to be advantageous if the non-consumable electrodes enclose an angle of 20xc2x0 to 70xc2x0, preferably 30xc2x0, with the electrode switched as a cathode preferably standing perpendicularly to the workpiece.
The wear and tear of the electrode connected to the plus pole and subject to higher stresses may be kept at a very low level if this electrode has a substantially blunt free end projecting from the nozzle in which it is held, and the non-consumable electrode connected to the minus pole has a substantially conical free end projecting from the nozzle in which it is held.
It must be ensured that the higher stressed electrode is cooled sufficiently well, and in a simple arrangement for this purpose, the device comprises a cooling conduit in the receiver, the cooling conduit having an inlet and an outlet, the non-consumable electrodes being held in the receiver in holders, the holders passing through chambers which are connected to each other by the cooling conduit, and the chamber through which the holder for the non-consumable electrode connected to the plus pole passes being connected to the inlet.
A device wherein at least the nozzle holding the non-consumable electrode connected to the minus pole is made of an electrically well-conducting material, preferably further comprises a high-resistance resistor having a resistance of 103 to 106 ohms, which connects this nozzle to the pole connected to a workpiece to be welded. This allows achieving an ionization of the plasma gas flowing from the nozzle in the zone between the electrode and the nozzle as a result of a high-frequency arc-over. This produces the ignition of an arc between the electrode and the workpiece as a result of the applied direct voltage. This leads to a substantial protection of the plasma torch because the same is not encumbered by the otherwise common pilot arc.
As a result of the ionization by the high-frequency arc-over, which imposes only very low thermal stressed on the nozzle, it is also possible when using helium as a plasma gas to easily ignite over larger distances between the electrode and workpiece of 10 mm, for example.
The use of a nozzle which is made from an electrically well-conducting material and its connection via a high-resistance electric resistor with the pole of the voltage source which is connected with the workpiece is also of advantage in devices in accordance with the invention in which the plus pole of the voltage source is connected with the electrode held in the nozzle.
It is desirable to produce a constriction of the plasma and avoid a divergence of the same due to the friction of the plasma in the air which emerges with a high speed so that a very high concentration of the energy is achieved. For this purpose, the non-consumable electrode connected to the minus pole has a substantially conical free end projecting from the nozzle in which it is held, and said nozzle has a conical surface extending substantially parallel to the conical free end, the cone angle being about 20xc2x0. Also, the device may further comprise cold gas conduits connected to a source of cold gas and distributed evenly and concentrically about an axis of a bore of at least one of the nozzles, the cold gas conduits being open at a nozzle end face from which a free end of the non-consumable electrode held therein projects, the axes of the cold gas conduits forming a conical generatrix whose apex is disposed before the free electrode end.